This invention relates generally to the recovery of rare earth metals. More specifically, the invention relates to the recovery of rare earth elements from fresh water to hypersaline solutions using liquid-liquid extraction methods.
The rare earth elements (REE) are among the most frequently cited critical materials for clean energy and high-tech manufacturing. The unique and varied properties of REE have led to their application in more consumer products than nearly any other element group. REE are mostly obtained from mining and processing of REE-enriched ores. However, mining is expensive and laborious with a significant environmental burden.
Aqueous byproduct or waste streams, both natural and industrial, are potential sources of the REE and other critical materials. With increasing global interest in geothermal energy, development of unconventional oil and gas resources (e.g. hydraulic fracturing of organic rich shales), and desalination technologies, large volumes of waste brines are being managed and processed at great expense. Development of technologies for recovery of valuable byproducts, such as the REE, from these waste streams could improve the economies of these technologies. Development of such technologies requires accurate determination of the source REE concentration in order to develop and implement recovery systems. However, precise quantitation of REE in complex matrices like brines is a significant challenge for conventional instrumentation such as inductively coupled plasma mass spectrometry (ICP-MS).
There exists a dearth of methodologies in the analytical literature for quantitation of REE in brines by ICP-MS. Many approaches have been applied for separation and concentration of REE from aqueous media including solid-phase extraction (SPE), co-precipitation (co-ppt), and liquid-liquid extraction (LLE). However, nearly all studies in the analytical literature have focused on fresh water or seawater matrices, neglecting hypersaline waters (i.e. more concentrated than seawater, ˜0.7 M NaCl). Despite this deficiency, approximately 14% of published measurements of REE in groundwater constitute brine samples (greater than 1 eq/kg ionic strength), with these analyses utilizing methodologies not explicitly validated for extreme salinity.
Commonly applied separation techniques such as SPE and co-ppt may lack the robustness necessary to analyze REE in hypersaline brines. For example, high dissolved organic carbon may lead to fouling of column-based SPE while high dissolved metal loads may lead to saturation of the surface sites responsible for REE binding. Oliveira, et al. ascribed diminished Zn recovery in 166% salinity produced water to competitive sorption of matrix cations on their iminodiacetate resin. Similarly, excessive cations in hypersaline solutions may screen the REE from sorption sites during co-ppt, a phenomenon noted by Nelson, et al. for Ra determination in produced waters from the Marcellus Shale by both BaSO4 and MnO2 co-ppt. Moreover, at the elevated pH necessary for SPE and co-ppt techniques, the formation of energetically favorable, neutral- or negatively-charged aqueous complexes of the REE (with both organic and inorganic ligands) can further limit REE-particle partitioning.
Liquid-liquid extractions are potentially robust to all of these conditions and represent an attractive alternative for REE separation from hypersaline solutions. Liquid-liquid extraction of REE from highly acidic solutions has been thoroughly studied for separation of lanthanides and actinides during nuclear fuel reprocessing; elevated pH is not required of LLE techniques. Moreover, electrolyte theory dictates that increased sample salinity should enhance chemical partitioning (through salting out of neutral/micellar REE-organic ligand complexes from the aqueous feed to the organic solvent) and physical phase separation (by collapsing the electric double layer of the organic droplets, hastening coalescence). A primary obstacle in extraction of hydrophilic metals to a hydrophobic, organic phase is the dehydration of the metal cations in the aqueous phase. However, increasing salt concentrations decrease the effective concentration of water in the solution available for hydration of the metal cations, improving the energetics of the extraction.